Printed circuit board for improving impedance of signal transmission lines

ABSTRACT

A printed circuit board includes a first ground layer and a second ground layer. The second ground layer is below the first ground layer. A signal transmission line is arranged on the printed circuit board. The first ground layer includes a chamber located below the transmission line. Therefore, the signal transmission line takes the second ground layer as a reference ground layer, so impedance of the signal transmission line is increased.

BACKGROUND

1. Field of the Invention

The present invention relates to a printed circuit board, and more particularly to a printed circuit board which can improve impedance of signal transmission lines.

2. General Background

IEEE 1394 is a very fast external bus standard that supports data transfer rates of up to 400 Mbps and 800 Mbps. A single 1394 port can be used to connect up to 63 external devices. To make computers that can fully take advantage of this standard is a challenge. In general, the thinner a printed circuit board (PCB) is, the better the signal transmission capability of signal transmission lines in the printed circuit board is which is needed to improve a computers' use of IEEE 1394. However, the thinner you make a PCB the less impedance the signal transmission lines have and thus, may fail to meet with the impedance requirements of IEEE 1394.

Referring to FIG. 3, a conventional printed circuit board 10 includes a first ground layer 13, a second ground layer 14, and a signal layer 15. Two differential pairs 11 and 12 are placed on the signal layer 15. The differential pair 11 includes a first differential signal line 110 and a second differential signal line 112, and the differential pair 12 includes a first differential signal line 120 and a second differential signal line 122. The differential pair 11 has a same dimension with the differential pair 12. Supposing that W is a width of the first differential signal line 110, H1 is a distance between the differential pair 11 and the first ground layer 13, S is a distance between the first differential signal line 110 and the second differential signal line 112, and Z is an impedance of the differential pair 11. The impedance of the differential pair 11 can be formulated as Z∝H1×S/W, that is, Z is directly proportional to H1 and S, and is inversely proportional to W. If a thickness of the printed circuit board 10 is reduced, the distance H1, between the differential pair 11 and the first ground layer 13, is decreased, while the W and the S remain unchanged, so the impedance Z, of the differential pair 11, is reduced. Therefore, the impedance of the differential pair 11 may not meet with the impedance requirements of IEEE 1394.

What is needed is a printed circuit board with improved impedance of signal transmission lines.

SUMMARY

An exemplary printed circuit board includes a first ground layer and a second ground layer. The second ground layer is below the first ground layer. A signal transmission line is arranged on the printed circuit board. The first ground layer includes a chamber located below the transmission line. Therefore, the signal transmission line takes the second ground layer as a reference ground layer, so impedance of the signal transmission line is increased.

Other advantages and novel features will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a printed circuit board in accordance with a first preferred embodiment of the present invention, together with two differential pairs;

FIG. 2 is a cross-sectional view of a printed circuit board in accordance with a second preferred embodiment of the present invention, together with two differential pairs; and

FIG. 3 is a cross-sectional view of a conventional printed circuit board, together with two differential pairs.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to FIG. 1, a printed circuit board 20 in accordance with a first preferred embodiment of the present invention includes a first ground layer 23, a second ground layer 24, a signal layer 25, and two media layers 26 and 27 including power layer, signal layer. The first ground layer 23 is below the signal layer 25, and the two media layers 26, 27 are located between the first ground layer 23 and the second ground layer 24. Two spaced differential pairs 21 and 22 are placed on the printed circuit board 20. The differential pair 21 includes a first differential signal line 210 and a second differential signal line 212, and the differential pair 12 includes a first differential signal line 220 and a second differential signal line 222. The differential pair 21 has a same dimension with the differential pair 22. The first ground layer 23 defines chambers 230 and 231 below the differential pairs 21 and 22 respectively. A width of each of the chambers 230 is equal to or more than that of each of the differential pairs 21 and 22, in order to expose the differential pairs 21 and 22 to the second ground layer 24. A length of each of the chambers 230 is generally equal to that of corresponding differential pairs 21, 22.

Supposing that a width of the first differential signal line 210 is W, a distance between the first differential signal line 210 and the second differential signal line 220 is S, a distance between the differential pair 21 and the first ground layer 23 is H1, a distance between the differential pair 21 and the second ground layer 24 is H2, and a width of the chamber 230 is B. Because the differential pairs 21 and 22 are located above the chambers 230 and 231, the second ground layer 24 is a reference ground layer of the differential pairs 21 and 22, instead of the first ground layer 23. Therefore, the impedance Z of the differential pair 21 is directly proportional to H2 and S, and is inversely proportional to W. The impedance of the differential pair 11 is increased because the distance between the differential pair 11 and the reference ground layer is increased. Thus, the impedance of the differential pair 12 is also increased.

Referring to FIG. 2, a printed circuit board 20′ in accordance with a second preferred embodiment of the present invention is shown. The printed circuit board 20′ includes a first ground layer 23′, a second ground layer 24′, a signal layer 25′, and two media layers 26′ and 27′. The first ground layer 23′ is below the signal layer 25′, and the two media layers 26′, 27′ are located between the first ground layer 23′ and the second ground layer 24′. A difference between the second preferred embodiment and the first preferred embodiment is that a single chamber 230′ is configured below both differential pairs 21 and 22. A reference ground layer of the differential pairs 21 and 22 is the second ground layer 24′. So impedances of the differential pairs 11 and 12 are increased.

It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments. 

1. A printed circuit board having at least one signal transmission line arranged thereon, the printed circuit board comprising: a first ground layer comprising a chamber, the chamber located below the transmission line; and a second ground layer below the first ground layer, the signal transmission line taking the second ground layer as a reference ground layer, to increase impedance of the signal transmission line.
 2. The printed circuit board as claimed in claim 1, wherein the width of the chamber is equal to or more than the width of the signal transmission line.
 3. The printed circuit board as claimed in claim 1, wherein the at least one signal transmission line is two signal transmission lines spaced apart on the printed circuit board.
 4. The printed circuit board as claimed in claim 3, wherein the chamber comprises two spaced parts, the two parts are corresponding to the two spaced signal transmission lines respectively.
 5. The printed circuit board as claimed in claim 3, wherein the two spaced signal transmission lines are above the chamber.
 6. The printed circuit board as claimed in claim 3, wherein the two spaced signal transmission lines are differential pairs and each comprises a first differential signal line and a second differential signal line.
 7. A printed circuit board comprising: a signal layer, at least one transmission line arranged thereon; a first ground layer, a portion of the first ground layer below the signal transmission line being hollow; and a second ground layer below the first ground layer, the signal transmission line taking the second ground layer as a reference ground layer, to increase impedance of the transmission line.
 8. The printed circuit board as claimed in claim 7, wherein the signal transmission line is a differential pair and comprises a first differential signal line and a second differential signal line.
 9. A printed circuit board comprising: a signal layer, at least one transmission line arranged thereon; a first ground layer located below the signal layer and defining a continuous slit corresponding to said transmission line, a length of the slit generally equal to a length of said transmission line; a second ground layer below the first ground layer, said signal transmission line taking the second ground layer as a reference ground layer, to increase impedance of the transmission line; and a media layer located between the first ground layer and the second ground layer.
 10. The printed circuit board as claimed in claim 9, wherein a width of the slit is equal to or more than a width of said transmission line.
 11. The printed circuit board as claimed in claim 9, wherein the at least one signal transmission line is two signal transmission lines spaced apart on the printed circuit board. 